Diseases of the gastrointestinal tract are a common cause for dogs and cats to be presented to a veterinarian. In order to arrive at the most appropriate diagnosis, veterinarians need to employ diagnostic tests. While no diagnostic test is always positive in a patient with a certain disease and no diagnostic test is always negative in a patient without the disease, the clinician needs to strive to use those tests with the highest clinical accuracy possible. In addition to a high accuracy, diagnostic tests should also be economical and as minimally invasive as possible.